Thermal ink jet printheads have previously been formed in an area of a silicon substrate coated with silicon dioxide (SiO.sub.2). A resistive heating layer of an alloy of tantalum and aluminum (TaAl), for example, is disposed on top of the layer of SiO.sub.2. A layer of aluminum is deposited over portions of the layer of TaAl to leave remaining portions of the layer of TaAl exposed. The exposed portions of the layer of TaAl between the spaced portions of aluminum, which supply the current to the layer of TaAl, form resistors or heaters. The resistors or heaters form bubbles of ink when current flows through the layer of TaAl between the spaced portions of the layer of aluminum to heat the ink.
Protective layers are formed over the patterned aluminum layer and the exposed portions of the layer of TaAl. Previously suggested thermal ink jet printheads had a layer of silicon nitride (Si.sub.3 N.sub.4) deposited over the patterned aluminum layer and the exposed portions of the layer of TaAl by plasma enhanced chemical vapor deposition (PECVD). Next, a layer of silicon carbide (SiC) was deposited over the layer of Si.sub.3 N.sub.4 by PECVD. Finally, a layer of Ta was sputtered onto the layer of SiC.
Ink, which is transformed into bubbles by the heat of the resistors, overlies the Ta layer. Thus, the ink must be heated by the resistors through the protective layers of Si.sub.3 N.sub.4, SiC, and Ta.
The protective layers of Si.sub.3 N.sub.4, SiC, and Ta protect the resistors from cavitation due to the ink bubbles collapsing after being generated and from chemical attack such as corrosive effects due to the turbulent ink and vapor. However, as the thickness of the protective layers is increased, it is more difficult to heat the ink through the protective layers because the protective layers impede the dissipation of heat.
Accordingly, as the total thickness of the protective layers has increased, it has previously been suggested to use current pulse of long times (widths) to apply sufficient heat to the ink to form the droplets. Longer pulse widths imply a lower power density to the heater. Lower power density has been correlated to lower print quality. Therefore, these relatively long pulse widths may degrade print quality.
A crack in the protective layers may lead to a failure of a resistor or heater. As the number of the resistors or heaters failing increases so as to not produce satisfactory print quality, the printhead fails.